The occupation of Outer Mongolia by the Beiyang government of the Republic of China after the revocation of Outer Mongolian autonomy (Chinese: 外蒙古撤治) began in October 1919 and lasted until 18 March 1921, when Chinese troops in Urga were routed by Baron Roman von Ungern-Sternberg's White Russian (Buryats,[2] Russians etc.)

Although the Beiyang government abolished the autonomy of the Bogd Khanate in Outer Mongolia and then expanded its occupation to include Uryankhay Krai (Tuva), it was unable to consolidate its rule over both regions.

[4] This claim was provided for in the Imperial Edict of the Abdication of the Qing Emperor signed by the Empress Dowager Longyu on behalf of the six-year-old Xuantong Emperor: "[...] the continued territorial integrity of the lands of the five races, Manchu, Han, Mongol, Hui, and Tibetan into one great Republic of China" ([...] 仍合滿、漢、蒙、回、藏五族完全領土，為一大中華民國).

Since 1918, Outer Mongolia was threatened by the Russian Civil War, and in summer 1918 asked for Chinese military assistance, which led to the deployment of a small force to Urga.

[9] Meanwhile, some Mongol aristocrats had become more and more dissatisfied with their marginalization at the hands of the theocratic Lamaist government, and, also provoked by the threat of the Outer Mongolia's independence from the pan-Mongolist movement of Grigory Semyonov in Siberia, were ready to accept Chinese rule by 1919.

[14] The prospect of ending Mongol autonomy and having Chinese troops stationed in Niialel Khuree, Altanbulag, Uliyasutai, and Khovd was permitted by the Mongolian government in response to the Japanese-backed Buryatia pan-Mongol movement.

[15] An ally of the Chinese government, the Qinghai-born Monguor Gelugpa Buddhist Lama leader Sixth Janggiya Khutughtu was against the autonomy of Outer Mongolia.

[16][17][18] The invasion of Mongolia was the brainchild of Chinese Prime Minister Duan Qirui, who engineered China's entry into World War I.

Duan gave control of it to his right-hand man, Xu Shuzheng, a member of the pro-Japanese Anhui clique in the Chinese government.

These data are not confirmed by archival documents which indicate that the occupation of Outer Mongolia was initiative of the Anhiu-led Chinese government caused by weakening of Russia by revolution and civil war.

[29] In February 1920, Xu presided over a very humiliating ceremony in which Bogd Khan and other leaders were forced to kowtow before him and the Five Races Under One Union flag.

That event marked the beginning of active resistance against Chinese rule, which coalesced into the Mongolian People's Party.

The invasion had caused alarm for Zhang Zuolin, the powerful warlord of Manchuria, who was upset that such a large army was moved so close to his territory.

[39][40] The plundering Inner Mongol Chahars were recruited by the Chinese High Commissioner Wu Tsin Lao with the deliberate knowledge that they would engage in looting.

[43] In October, the White Russian Baron Roman von Ungern-Sternberg[44] swept into Mongolia from the north, fought many battles against the Chinese garrison stationed in Urga, and captured it in February 1921.

[50][51][52] A joint MPP-Red Army expedition led by Soviet Red commanders and Damdin Sükhbaatar defeated Ungern in August.

[70] The Paris Peace Conference was attended by representatives from the "Dauria Government" of the pan-Mongol initiative established in February 1919 by Semyonov.

[76] After the Asiatic Cavalry Division drove out the Chinese troops occupying Urga, Mongolia had regained its independence in February 1921.

[78] The Chinese Army in June 1921 defeated a 350 strong White Russian unit, led by Colonel Kazagrandi, most of whom died in battle although 42 became prisoners.

[34] For China, the occupation indirectly led to the permanent breakup of the Beiyang Army and the fall of strongman Duan Qirui.

[82] Informal relations were established between Mongolia and Taiwan via trade offices in Ulan Bator and Taipei but without formal diplomatic recognition.

[84][85] The Soviets divided the Mongols away from the Tuvan and the Buryat peoples, though they had been separate during the preceding Russian Empire and the time of the Uryankhay Republic.

[86] During Soviet rule, Bolshevik officials conducted an anti-Buddhist campaign in Buryatia, executing many in a cultural and intelligentsia purge.